Originally Posted by Little-Fizz
In a 28 gallon I've got 5 serpae tetras, 5 cory cats, and 4 swordtails. Plus three shrimp and a snail.
Allrightythen. Let's break this down. 28 gallons = 28 inch equivalents, or up to 56 if heavily planted (very heavily - if you get asked how your plants grow so well or why your tanks looks like a jungle, then you're heavily planted, otherwise, 28 it remains) and established for more than 6 months. Adjust downward by 20% if that's one of the 28 gallon high tanks that has the same footprint as a 20 high, so somewhere between 22 and 56 inch equivalents of bioload. Overfiltering and frequent water changes can stretch that a little.
Serpaes are 1.5 inch = 1 inch equivalent (tetras are generally low bioload, but serpaes are active and nippy and need some extra space), so 5" equivalent.
Cories are 1 to 1, and can (for females of the more common species) get up to 3". Call it 2.5" on average = 12.5" equivalent of Cories
Swordtails get up to 4", slightly more for females. They are mildly aggressive, so they need extra space. Bioload increases exponemtially with length, so I would say 1" of swordtail = 2" equivalents (that's generous), meaning you have up to 32" equivalents of fish there at full growth. And they will breed if you have a male in there (or kill each other if you have more than 1 male in there).
Now, if your tank is heavily planted, then some fry will survive, and if you're not overstocked already, you will be. If you're not heavily planted, then the fry are less likely to survive, and you'll be overstocked as the swords get over about 2 1/2".
This isn't even counting shrimp and snails which can have a widely variable impact (think cherry red at 1" vs african vampire shrimp at 8", or pond snails which will breed vs nerites which probably won't vs apple snails which are a good 10 times larger in all dimensions.)
So I'd say make sure you have filtration for up to 80 gallons or so, get some plants in there if you don't already, keep your snail population down, and don't add anything else.